The search for a clean, efficient internal combustion engine system has focused considerable at ention on the benefits of hydrogen as a fuel. The absence of carbon in this fuel virtually eliminates hydrocarbon and carbon monoxide emissions. The extremely low lean flammability limit of hydrogen allows lean combustion with low NO.sub.X production and increased engine efficiency. One of the disadvantages of hydrogen fueled engines is the reduction in maximum power as the gaseous fuel displaces some of the air during the intake stroke.
Hydrogen fueled engines have not come into use due to the difficulty of storing hydrogen onboard a vehicle; either as a compressed gas or as a cryogenic liquid. Storage of hydrogen as a hydride appears promising, but the technology is not sufficiently developed yet and the concept has an inherent high weight penalty associated with it.
An alternative to hydrogen storage is to generate the hydrogen onboard the vehicle on demand from a stored liquid fuel. The first choice for such storable liquid fuel is of course gasoline itself. A compact onboard hydrogen generator has been developed based on the partial oxidation of gasoline with air. (U.S. Pat. No. 4,003,133 issued to Houseman et al.) However, the hydrogen gas generated from partial oxidation of gasoline contains undesirable amounts of carbon monoxide, together with nitrogen diluent. Also, during the conversion of gasoline to hydrogen 22% of the energy content of the fuel is released as sensible heat which cannot be utilized. Also, the hydrogen generator operates at high temperatures (980.degree.-1050.degree. C.), which requires a long start-up time and special materials of construction.
Gasoline is also undesirable since it is subject to increasing fuel costs and to diminishing oil reserves. As a result, there is growing interest in the search for alternative fuels to reduce the dependency on expensive oil imports. Long before today's energy crisis, alcohol fuels were proposed as gasoline-blending compounds for use in internal combustion (IC) engines. Two alcohol compounds, ethanol and methanol, are still receiving continued attention. Racing cars, for example, use alcohol blends because of their increased power relative to gasoline.
Ethanol can be produced by fermentation from agricultural products such as grain, cane, molasses, potatoes, and mannite, a tropical plant. Methanol can be manufactured from a large variety of materials, including wood, seaweed, municipal wastes, residual oil, peat, and coal. Because of the abundant coal reserves in the United States, the future supply of methanol seems more promising than that of ethanol. Further, methanol can be produced relatively easily from coal gasification products with a high thermal efficiency and at a reasonable price. As such, methanol appears to be attractive from an energy self-sufficiency point of view for use as a storable liquid fuel for generating hydrogen.
The direct decomposition of 1 mole of methanol into 2 moles of hydrogen and 1 mole of carbon monoxide represents a convenient cycle for generating hydrogen-rich gas from liquid methanol: EQU CH.sub.3 OH.sub.(1) .fwdarw.2H.sub.2(g) +CO.sub.(g)
Methanol contains a lower heating value of 19,910 kj/kg (8560 Btu/lb) while the corresponding hydrogen and carbon monoxide products from the reaction CH.sub.3 OH.fwdarw.2H.sub.2 +CO contain a combined lower heating value of 23,840 kj/kg (10,250 Btu/lb). The 20% increase in heat content of the dissociated methanol products is derived from the energy which is consumed in the cleavage of hydrogen-carbon and hydrogen-oxygen chemical bonds to produce hydrogen and carbon monoxide.
To facilitate this endothermic chemical reaction and further enhance the system energy gain, engine exhaust gas heat can be utilized which usually has a temperature range from 200.degree. C. to 650.degree. C. Thermodynamic equilibrium would predict carbon formation and very little hydrogen and carbon monoxide production in the normal engine exhaust temperature range. This is illustrated in FIG. 1. Carbon soot not only decreases the energy efficiency of the methanol decomposition, but also causes clogging of carburetor jets and float mechanisms. An example of such a carbon soot producing methanol reactor using exhaust gas as a source of heat is given by Dimitroff, E. and Vitkovits, J. A. of the Southwest Research Institute in a paper presented at the 1976 Spring Meeting of the Central States Section of the Combustion Institute.
A catalyst can be utilized to inhibit the formation of carbon and to facilitate the decomposition reaction at lower temperatures by controlling the mechanism by which the methanol molecule reacts. Furthermore, in an engine which would be combusting hydrogen/carbon monoxide, the engine would probably be operated at leaner equivalence ratios (1, 2) which could be as lean as .phi.=0.5. This would lower the engine exhaust gas temperatures even further demanding the activity of the catalyst to be high and over a wide temperature range as well as an effective exchange of heat from the exhaust gas into the methanol catalyst bed.
The catalyst must be capable of operation at temperatures up to 650.degree. C. (1200.degree. F.), under pressures to 1034 kPa. A minimum conversion of 80% may be considered acceptable under maximum flow conditions depending on the engine and duty. The catalyst must be able to withstand condensation of liquid methanol on the catalyst particles which may occur during cold start without spoiling or crumbling. High structural integrity during thermal cycling without carbon formation are also necessary catalyst properties. Selectivity of methanol decomposition (to hydrogen and carbon monoxide) over dehydration to dimethyl ether (2CH.sub.3 OH.fwdarw.CH.sub.3 OCH.sub.3 +H.sub.2 O) and limited activity for methanation due to product recombination (3H.sub.2 +CO.fwdarw.CH.sub.4 +H.sub.2 O) under the various operating conditions are also important considerations.